Examples of a conventional technique of determining a situation of a vehicle around a user vehicle include the following techniques.
In the first conventional technique, a surrounding sound is converted into a sound pressure level signal. An absolute quantity of the sound pressure level signal in a specific frequency band is compared with a determination level. It is determined, using the comparison result, whether or not a vehicle is present around a user vehicle. In addition, it is determined whether or not another vehicle is approaching, based on a time dependency of the sound pressure level signal (see Patent Literature (PTL) 1, for instance).